Novel Steel Combat Helmet and Method of Production Thereof

ABSTRACT

The invention relates to helmets designed to protect their wearers from ballistic impacts and shrapnel. Such helmets are well-known, and are the current standard for law enforcement and military use alike. Whereas such helmets used to be made solely of steel, combat helmets are today manufactured primarily from polymer composites such as aramid and ultra-high molecular weight polyethylene, and metal helmets have been largely superseded in common use, if not considered entirely obsolete, since the 1980s. The present invention, described herein, allows for the production of a modern steel helmet which is lightweight and exhibits excellent ballistic performance—performance which is comparable if not superior to that exhibited by the best polymer composite helmets of our day.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of, and incorporates by reference for all purposes, the following U.S. provisional applications: U.S. Provisional Application No. 62/771,182, filed Nov. 28, 2018, entitled NOVEL STEEL COMBAT HELMET AND METHOD OF PRODUCTION THEREOF; U.S. Provisional Application No. 62/897,946, filed Sep. 19, 2019, entitled HIGH-STRENGTH STEEL COMBAT HELMET AND METHOD OF PRODUCTION THEREOF.

FIELD OF THE INVENTION

This invention relates generally to anti-ballistic protective headgear and helmets. More specifically, this invention relates to modernized steel combat helmets which exhibit good ballistic, blunt, and blast performance.

BACKGROUND OF THE INVENTION

Armor Devices:

Ballistic helmets are generally used to absorb the impact from firearm-fired projectiles and fragments from explosions and fragmenting munitions. Helmets are the most basic, fundamental component of armor, and have been in use since the dawn of civilization. From the Sumerians, through the Ancient Greeks and Romans, and to the peak of plate armor's development in the 16th century, the helmet was considered a vital component of the warrior's equipage.

The plate armor and helmets of the 14th-17th centuries were increasingly designed to withstand ballistic impacts, and grew thick and heavy over time. Some of the helmets of that period weighed more than twenty pounds. In 1587, Michel de Montaigne noted that “today the officer is so heavily armed that by the time he becomes thirty-five his shoulders are completely humpbacked.” In a separate account of roughly the same period, Montaigne noted that “Alexander, the most adventurous captain that ever was, very seldom wore armour, and such amongst us as slight it, do not by that much harm to the main concern; for if we see some killed for want of it, there are few less whom the lumber of arms helps to destroy, either by being overburthened, crushed, and cramped with their weight, by a rude shock, or otherwise. For, in plain truth, to observe the weight and thickness of the armour we have now in use, it seems as if we only sought to defend ourselves, and are rather loaded than secured by it.” This was, at the time, a popular view—one echoed by many other writers from that period. Very shortly after it was written, the Swedish, who pioneered the use of mobile and very lightly-armored troops and artillery, attained great military victories in the Thirty Years' War, and helmets and body armor swiftly began to disappear from the battlefield.

So it was that at the outset of World War One, soldiers were issued no protective armor of any type. In a perhaps apocryphal story, it is said that Intendant-General August-Louis Adrian of the French army was speaking to a wounded soldier, who told him that an inverted mess bowl worn under his cap saved him from a gravely serious head wound. General Adrian is said to have realized that protective headgear might save the lives of French soldiers, and experimented with a metal skull-cap (calotte métallique, cervelière) to be worn under the standard-issue military hat (kepi). This skull-cap was 0.5 mm thick, made of mild steel, and offered limited protection against fragments, shrapnel, and blunt impact—yet its use did produce positive results, as it led to a measurable decrease in the number of head-wound casualties. Between December 1914 and February 1915, 700,000 of these skull-caps were made, and 200,000 of them were issued to the ranks.

The skull-cap caught the attention of General Adrien's superior, Marshal Joseph Joffre, and in April 1915 Joffre ordered General Adrien to set to work on a more sophisticated solution. The Paris Fire Brigade's helmet was rapidly adapted for wartime use and designated the “Casque Adrian.” It was 0.7 mm thick, made of mild steel, and weighed roughly 1.8 pounds. Initially developed only for infantry, it was quickly distributed to all branches of the military. By the end of 1915, over 3 million Adrian helmets had been issued to the ranks.

The British and Germans quickly followed in the footsteps of the French. The former issued a helmet designed by John Brodie in 1916. Initially made of mild steel, it was quickly recognized that Hadfield Manganese steel—an austenitic manganese steel containing 13% manganese and 1.2% carbon, which was patented in 1883—would offer significantly better ballistic protection. Hadfield Manganese steel is a strain-hardening steel which rapidly transforms from ductile austenite to strong martensite when it's subjected to shock or impact. Austenitic Hadfield steel is also nonmagnetic, which was important for soldiers who needed to rely upon compass readings. The Brodie, as-issued, was roughly 0.85 mm thick, and weighed approximately 1.3 pounds in total. It was primarily designed to protect from falling shrapnel, so it was built with a broad brim, reminiscent of the medieval “kettle hat.”

The Brodie helmet formed the basis of subsequent American helmet development. In 1917, the American Expeditionary Forces of WWI were equipped with Brodie helmets; first from a purchase of 400,000 British surplus units, and then from an American-made copy of those helmets, designated the M1917 Kelly. Nearly 3 million American M1917 helmets had been produced before the end of the war. In 1941, the M1917 was replaced with the M-1 helmet, which was larger, bowl-shaped, and eschewed the Brodie's broad brim in favor of more protection for the sides and back of the head. The M-1 helmet shell was 0.94 mm thick and, like the M1917, was made of Hadfield Manganese steel. It weighed approximately 2.25 pounds without the liner and chinstrap. Millions of M-1 helmets were produced, and they were the standard-issue helmet for US military forces until 1983.

The Personnel Armor System for Ground Troops (PASGT) helmet was tested in the 1970s and fielded in the early 1980s. These PASGT helmets weighed between 3.1 pounds (size XS) to 4.2 pounds (size XL)—so were therefore heavier than the Ml. They did, however, provide increased ballistic protection.

Unlike all previous helmets going back to the Sumerians, the PASGT was not made of metal. While the Vietnam War was raging, DuPont (E. I. du Pont de Nemours and Company, Wilmington, Del.) polymer chemist Stephanie Kwolek was experimenting with lightweight yet strong fibers for use in reinforcing automobile tires. To this end she was working with the stiff polymers poly-p-phenylene terephthalate and polybenzamide, when she noticed that the reaction product of the two polymers displayed some unusual properties. As she later explained:

“The solution was unusually (low viscosity), turbid, stir-opalescent and buttermilk in appearance. Conventional polymer solutions are usually clear or translucent and have the viscosity of molasses, more or less. The solution that I prepared looked like a dispersion but was totally filterable through a fine pore filter. This was a liquid crystalline solution, but I did not know it at the time.”

Unusual-looking chemical solutions of that sort were not often pursued—they were typically discarded. And indeed Kwolek went on to mention that “I think someone who wasn't thinking very much or just wasn't aware or took less interest in it, would have thrown it out.” But she took an interest in the unusual chemical polymer solution, so it was spun on a spinneret, and the mechanical properties of the resulting fibers were tested. It quickly became apparent that this new aramid fiber was three times as strong as nylon and possessed a far higher modulus and heat resistance. And, despite its high modulus, the fibers were so fine that woven fabrics derived from it were flexible and draped like nylon.

The military quickly seized upon this new material, which was first known as PRD-29 and PRD-49, and later as DuPont Kevlar® 29 and Kevlar® 49. Kevlar 29 was judged the superior grade for ballistic protection, and flak vests were made which offered significantly greater protective ability than those made of nylon and Doron, at the same weight. It was also used in helmets which were comprised of resin-bonded laminates of Kevlar 29. These helmets were eventually issued as the PASGT system helmet.

The PASGT helmet, like the Casque Adrien before it, became the prototype which informed all subsequent combat helmet development. Compared to the M-1, the PASGT—and the ACH helmet which came after it, which also has an aramid shell, is of a similar weight, and offers similar performance—does indeed offer superior protection from fragments and shrapnel. And, unlike the M-1, it proved capable of defeating virtually all ball rounds fired from handguns and submachine guns.

Aramid-based combat helmets sold and issued today are commonly rated to the NIJ Standard-0106.01, “Ballistic Helmets,” Type II. This specifies that they must defeat 0.357 Magnum jacketed soft point (JSP) bullets with nominal masses of 10.2 g (158 gr) and measured velocities of 425±15 meters (1395±50 ft) per second—as well as 9 mm full metal jacket (FMJ) bullets with nominal masses of 8.0 g (124 gr) and measured velocities of 358±15 m (1175±50 ft) per second.

Less frequently, but still quite commonly, aramid-based combat helmets sold and issued today are rated to the NIJ's Level Ma ballistic standard, which specifies that they must resist penetration from 0.357 SIG FMJ Flat Nose (FN) bullets with a specified mass of 8.1 g (125 gr) and a velocity of 448±9.1 meters per second (1470±30 ft/s) and from 0.44 Magnum Semi Jacketed Hollow Point (SJHP) bullets with a specified mass of 15.6 g (240 gr) and a velocity of 436±9.1 m/s (1430±30 ft/s).

The M-1 helmet would stop none of the aforementioned ballistic threats.

And although it would be fair to note that pistols are not commonly encountered on the battlefield, fragmenting munitions are still extremely prevalent—in every war since WWI, they have accounted for far more casualties than all small arms. With that in mind, this must be noted: The velocity at which a projectile has a 50 percent probability of perforating the armor called the ballistic limit or V50 of the armor for the particular projectile. Against the 17 grain fragment-simulating projectile, the PASGT was required to have a V50 of 2000 feet per second (MIL-H44099A, para. 3.5.2) with a 95 percent confidence level. The M-1, in contrast, had a V50 of under 1000 feet per second against the 17 gr. FSP.

Helmets made of ultra-high molecular weight polyethylene (UHMWPE) fiber laminates—an advanced polymer composite which is of lower density than aramid and can exhibit better performance at an equal or lesser system weight—have exhibited V50 values of over 3000 feet per second against the 17 gr. FSP. These UHMWPE helmets are also capable of stopping lead-core rifle rounds at reduced velocities, which is a remarkable feat.

It should be obvious, therefore, that the PASGT helmet and its successors offer better protection than that last of the steel helmets, the M-1. Yet polymer composite helmets have shortcomings of their own, and these are frequently very serious shortcomings.

First of which is environmental durability. UHMWPE helmets begin to degrade at temperatures hardly over 70° C. —a fact which limits their use in extreme environments, may prevent their storage in vehicles, and so forth.

Second of which is cost. Aramid and UHMWPE helmets are complicated and expensive to manufacture. The M-1 helmet shell, in 1943, cost the US Government approximately $1.05 per unit, which equates to roughly $15.05 in 2018 dollars—far less than a pair of boots, and not much more than the $0.73 water canteen. In 2010, the ACH helmet cost the government $250 per unit. The latest generation of UHMWPE helmets are even more expensive on a per-unit basis; police departments across America and worldwide can expect to pay from $500/unit to well over $1000/unit for high-quality SWAT helmets.

Most importantly, polymer composite helmets typically exhibit very poor structural rigidity. Not only are they compressible, they delaminate and bulge when they are impacted by a projectile. This is commonly called “backface deformation”, and is the characteristic impact response of a helmet which has a very high resistance to penetration combined with low rigidity—in such a case, the helmet will dissipate the projectile's kinetic energy via the plastic deformation of its shell. When the shell then comes into contact with its wearer's skull, head injuries frequently result, which include but are not limited to the following: Skull fracture, brain contusion, dura contusions, rotational shear injury, cavitation caused by differential acceleration of the skull and its contents, and cervical spinal damage. Indeed, as Rafaels et al. write in Injuries of the Head from Backface Deformation of Ballistic Protective Helmets Under Ballistic Impact (2015):

“ . . . it is clear that behind armor blunt trauma can result in significant injuries, namely linear fractures far from the impact location and depressed fractures near the site of impact. Because the level of injury is directly related to the helmet's material and the distance between the helmet and the skull, as helmet design moves toward lighter materials, the amount of helmet deformation needed to prevent bullet penetration will increase for a given standoff. Therefore, the risk of severe injuries due to BABT will also increase. This problem needs to be addressed in future helmet design. [ . . . ] The results of this study demonstrate a high risk of skull fracture due to BABT and necessitate the prevention of BABT as a design factor in future designs of helmets and other protective gear”

Thus there is clearly a need for a durable, inexpensive, high-performance combat helmet, which minimizes the risk of behind-armor blunt trauma. The present invention provides just such a helmet, along with the means for its production.

Hadfield Steel is not the final word in ballistic steel. As far back as 1917, United States manufacturers had attempted to produce helmets from high-hardness chrome-nickel-vanadium steel. Unfortunately, these methods failed; the steel was too difficult to form, and inevitably cracked during the cold-stamping operation. The US had to resort to helmets of Hadfield's high-manganese steel simply on account of the fact that it could be made cheaply and rapidly with that era's means of production. Yet Hadfield Steel wasn't even the best ballistic steel of its time, to say nothing of today. It is surprising, therefore, that all US military helmets, from WWI until the introduction of the PASGT, were made of Hadfield Manganese Steel.

Though not intuitively obvious, recent advances in steel processing allow for ultra-high-strength steels to be formed into helmet shells. Hot stamping, in particular, is a new and revolutionary forming process, which is typically used for ultra-high-strength steel automotive structural body components, such as bumper beams, roof pillars, side-impact protection beams, and floor and roof reinforcements. These safety-critical structural body components are designed to form a safety-cell around the passenger compartment, in order to maintain a survival space for the vehicle occupants. Clearly, these structural components must resist deformation at high loads and strain rates! Moreover, due to an increasing trend towards fuel economy, they must be as light as possible. Hot-stamped steel and hot-stamping alloys are, thus, ideally suited towards use in body armor.

Conventional cold-drawing, superplastic forming, hydroforming, and stretch-forming techniques, all well known to those familiar in the art, are now capable of producing helmets of extremely high-strength and high-toughness steel. At thicknesses which far exceeds that of any steel helmet ever produced by the military—which is to say, at thicknesses of 1.65 to 2.2 mm—steel helmets made via these techniques exhibit ballistic performance comparable to modern aramid or UHMWPE helmets. A prototype steel helmet made via conventional deep-drawing techniques, at a uniform 1.85 mm thickness, exhibited superior performance to the PASGT helmet at a lighter weight. It exhibited a V50 against the 17-grain fragment simulating projectile (FSP) of well over 2000 feet per second.

Another recent advance in steel processing which would increase the ballistic resistance of a steel helmet is to subject the exterior of the helmet shell to a plastic deformation treatment, such as shot peening, severe shot peening (SSP), surface mechanical attrition treatment (SMAT), or ultrasonic nanocrystalline surface modification (UNSM). These treatments induce the formation of a hard nanocrystalline surface layer, which increases the hardness, tensile strength, and structural rigidity of the steel helmet precisely where these traits are most necessary: On the helmet's strike face, where it would come into contact with a projectile.

In one or more embodiments of the invention, further surface hardness is required, thus the steel helmet shell is coated with a hard ceramic material. Suitable coatings include diamondlike carbon (DLC), titanium nitride (TiN), aluminum titanium nitride (AlTiN), aluminum chromium nitride (AlCrN), rhenium diboride (ReB2), silicon nitride (Si3N4), and their combinations and composites. Coating with these materials can be accomplished via thermal spray, cold spray, vapor deposition, or other methods known to those versed in the art.

FIG. 1 depicts a view of a representative hot stamping production process. It illustrates 1 the sheet steel cutting and forming process. In 2 the sheet steel blank is furnace heated to 900-950° C. so as to achieve a homogenous austenitic microstructure. It is then rapidly transferred to the press, 3, where it is stamped (formed) into a helmet shell geometry. Owing to the high temperature austenitic microstructure that is maintained throughout the stamping process, the blank exhibits low strength (often ˜200 MPa), high ductility (often >50% true strain), plastic isotropicity, and thus ultimately exhibits very high formability. This enables even very high-strength steels, in thick sections of >2 mm, to be formed into hemispherical, helmet-like geometries.

Note that the press stage can vary widely. In some cases, a pre-formed blank, which has been cold pressed to 20-90% of its final shape, can subsequently be hot stamped to its final geometry. In other cases, the blank need not be heated in a furnace to an austenitic microstructure; instead, it can be resistance-heated in the stamping die at a very high rate. Other methods for hot stamping exist and may be utilized. The fundamental point is that the blank or pre-form is pressed to shape under heat, or in a heated condition, and subsequently cooled and hardened in the die.

Following the stamping stage depicted in 3, the unfinished shell can be trimmed to its final shape via laser-cutter, 4. It can also be tempered, if necessary, which is undepicted in the image owing to the fact that it is rarely necessary.

The trimmed and tempered shell can then be subjected to a plastic deformation treatment, e.g. in the SMAT chamber depicted in 5.

This process allows for steel helmets with an ultimate tensile strength of well over 2000 MPa, a surface microhardness from 500 Vickers to well over 800, and excellent ballistic performance.

Furthermore, this process allows for steel helmets which are cheap to manufacture. The materials costs are typically no more than $10 per shell, and can be under $5 per shell. Material costs are therefore low—and manufacturing costs are also generally low at high output levels. Stamping or hydroforming and plastic deformation treatments are extremely high-throughput production methods which can be run in bulk quantities suitable for rapid military production.

Lastly, and perhaps most importantly, this process allows for extremely stiff helmets that do not deform significantly upon impact. A prototype helmet of the current invention was tested in ballistic and fragmentation experiments. This prototype was 2 mm thick and exhibited an areal density similar to today's aramid and UHMWPE helmets—a size M mid-cut helmet shell would weigh under three pounds. Backface deformation following a high-velocity 9 mm impact was undetectable. The residual impact force a wearer would experience was measured at a minimum of 0 joules to a maximum of under 3 joules—in all cases, very far below even the most cautious injury threshold. The fragmentation performance of this shell exceeded the PASGT standard, with a V50 of over 2000 feet per second against the 17 gr. FSP. With optimization of alloy selection and surface treatment, performance can be improved further, to a V50 of over 2500 feet per second.

The combination of a stiff steel shell with modern high-performance padding systems enables the design of closely-fitting helmets. As steel doesn't deform significantly upon impact, and as multiple shell sizes can easily be manufactured via conventional and hot stamping processes, the steel helmet of the present invention can be made so that it provides a closer fit, with less of a standoff between the helmet and the wearer's head. This would offer enhanced stability, better weight distribution, and a smaller presented target area; as the standoff decreases, so does the area and weight of the helmet, with no decrease in protection, and the helmet becomes a smaller target. These parameters directly influence both soldier acceptance and the protective capability of the helmet. Even if the steel helmet shell rests less than 10 mm from the wearer's head, which is to say that the helmet's pads are relatively stiff and roughly ½″ thick, the helmet would be fully compliant with Standard 0106.01, “Ballistic Helmets,” Type II, and would exhibit an average backface deformation value of less than 10 mm at all fair-hit impact locations.

As previously noted, there is a pressing and as-yet unmet need for the helmet of the present invention. Mild traumatic brain injury has become so common in recent years that it is sometimes called the War on Terror's “signature wound”—and it is indeed among the most common traumatic injuries sustained by military personnel. The optimized, modern steel helmet of the current invention would offer enhanced protection from this signature injury.

In one or more embodiments of the present invention, the steel alloy used in the helmet of the present invention is a hot-stamping alloy such as 22MnB5, 37MnB5, 38MnB5Nb. These are low to moderate-carbon, low-alloy steels, which include >1% Mn, >0.2% Si.

In one or more embodiments, the steel alloy used in the helmet of the present invention is in the Fe-(15-30)% Mn alloy system with additions of C, Al and/or Si to fully stabilize the f.c.c. phase and control stacking fault energy. In a particularly preferred embodiment of the present invention, stacking fault energy is controlled within the narrow range of 15-30 mJ/m².

In one or more embodiments, the steel alloy used in the helmet of the present invention is an ultra-high-strength martensitic armor steel, with a tensile strength over 2000 MPa, and a nominal composition:

0.4 C

4 Ni

1.5 Mn

0.9 Cr

0.6 Mo

1 Si

Bal Fe

In one or more embodiments, the steel alloy used in the helmet of the present invention is a high-toughness low-alloy steel with the nominal composition:

0.25 C

1.6 Mn

0.9 Si

Bal Fe

It is noteworthy that all prospective helmet alloys include substantial amounts of the alloying element Mn, as well as Si and/or Al.

An example of a steel helmet provided for by the present invention is comprised of a 1.65 mm steel shell manufactured via deep drawing, of the composition 0.25 C 1.6 Mn 0.9 Si, lightly coated with sealant and/or paint to military specifications. The interior of this helmet is lined with hook-and-loop fasteners, to which foam pads are attached. A harness or retention system is bonded via a polymer adhesive to the helmet shell's interior, or is attached via bolt holes drilled or laser-cut into the helmet's shell.

It is noteworthy that steel helmets over 1.1 mm in thickness have never been issued to troops, and, in the vast majority of instances, steel helmets in military service were well under 1 mm thick. The steel helmet of the present invention represents a significant improvement in terms of metallurgy, in terms of processing, and ultimately in terms of performance—in part, if not only, because of the enhanced thickness it is capable of attaining. What's more, a steel helmet with pads instead of webbing has never before been seen or issued to troops, yet would represent a tremendous improvement over all old steel helmets and most composite helmets both in terms of comfort and in terms of performance. For it is well known that blast waves often travel underneath the helmet, between the shell and a harness comprised of leather or fabric webbing, and the blast wave typically becomes amplified in those tight spaces. The best way to prevent or mitigate blast wave “underwash,” and thereby improve the functional performance of the helmet, is to remove the airgap between the helmet shell and the wearer's head. This is best achieved with a key manifestation of the present invention—a low-profile, high-performance steel helmet that fits closely to the wearer's head, combined with a padded liner.

It should be noted that, when employed in the present disclosure, the terms “comprises,” “comprising,” and other derivatives from the root term “comprise” are intended to be open-ended terms that specify the presence of any stated features, elements, steps, or components, and are not intended to preclude the presence or addition of one or more other features, elements, integers, steps, components, or groups thereof.

While it is apparent that the illustrative embodiments of the invention herein disclosed fulfill the objectives of the present application, it will be appreciated that numerous modifications and other embodiments may be devised by one of ordinary skill in the art. Accordingly, it will be understood that the appended claims are intended to cover all such modifications and embodiments, which come within the spirit and scope of the present invention. 

What is claimed is:
 1. A method for manufacturing a high-strength ballistic-resistant steel helmet shell, comprising the steps of: a) Preparing a steel blank by punching or cutting a steel plate, a steel sheet, or a section bar to appropriate dimensions. b) Pre-treating the blank by austenitization or cold-drawing followed by austenitization. c) Forming the helmet shell by pressing the heated and austenitized pre-form in a die. Once the draw depth is reached the part is hardened in the die by targeted cooling. d) Finishing the forming process by tempering and quenching, to whatever extent necessary for optimal ballistic resistance, and then trimming the formed blank to its final shape. e) Subjecting the exterior of the helmet shell to shot peening, severe shot peening (SSP), surface mechanical attrition treatment (SMAT), or ultrasonic nanocrystalline surface modification (UNSM), to induce the formation of a hard nanocrystalline surface layer, to whatever extent necessary for the degree of ballistic resistance required.
 2. A steel helmet shell, produced via hot stamping, press hardening, hydroforming, stamping, stretch forming, or super-plastic forming, wherein the finished shell is greater than 1.6 mm in thickness, and wherein the alloy contains >0.20% Al or Si, and more than 0.5% but less than 3% Mn, in addition to >0.18% carbon and other alloying elements, and wherein the finished helmet shell has a tensile strength over 1400 MPa.
 3. The steel helmet shell of claims 1 and 2, wherein the exterior surface of the shell is coated with an hard corrosion-resistant ceramic material via physical vapor deposition, chemical vapor deposition, thermal spray, or cold spray.
 4. A steel combat helmet comprised of the steel shell of claim 1 or 2, a harness, a padding system, and optional points for the attachment of accessories, wherein the stand-off distance between the shell and the wearer's skull is less than 16 mm, and wherein the helmet is nevertheless compliant with the NIJ Standard 0106.01, “Ballistic Helmets,” Type II. 